The present invention broadly relates to infrared intrusion detectors.
Generally speaking, the present invention relates to an infrared intrusion detector having an infrared sensor enclosed by a housing and having an optical arrangement which directs to the sensor infrared radiation entering the housing from specific reception zones through an entrance window which is permeable or transparent to infrared radiation and also having an evaluation circuit which is connected with the sensor and which emits a signal if the output signal of the sensor changes in a specific manner. The housing comprises an infrared radiation source which is designed and disposed in such a manner that the radiation therefrom impinges on or irradiates the sensor after penetrating or traversing the entrance window. The evaluation circuit is designed such that it additionally emits a signal if the sensor receives from the radiation source radiation which is diminished or attenuated in a specific manner.
In other words, the present invention relates to an infrared intrusion detector which comprises a housing having an entrance window defining reception zones or regions of the detector and permeable or transparent to external infrared radiation, an infrared sensor for generating an output signal enclosed in the housing, an optical arrangement for directing external infrared radiation entering the housing through the entrance window from predetermined ones of the reception zones or regions to the sensor, an evaluation circuit connected to the sensor for generating a first alarm signal in response to a predetermined type of change in the output signal, an infrared radiation source for emitting checking infrared radiaton contained in the housing and constructed and arranged such that the checking infrared radiation impinges upon or irradiates the sensor after traversing the entrance window, the evaluation circuit being constructed for additionally generating a second alarm signal in response to a predetermined degree of attenuation of the checking infrared radiation period or interval.
Such infrared intrusion detectors are known for example, from the British Patent Application No. 2,141,228, published Dec. 12, 1984 and serve to detect an object which has penetrated into a supervised or monitored area, e.g., an intruder, by means of the infrared radiation emitted or altered by the latter and to trigger an alarm signal by means of an evaluation circuit. In order to protect the optical arrangement and the sensor of such detectors from damage or dust, and in order to place the detector in an inconspicuous position in the monitored space, in such an arrangement the housing of the detector is closed in the direction of irradiation by an infrared-permeable window which is permeable or transparent to the radiation to be detected, e.g., the body radiation of a human being in the wavelength range around 10 .mu.m, e.g. within the range of 5 to 10 .mu.m. As a result of the additional infrared radiation source, the effect is achieved that the operative condition or state, i.e. the functionality of the detector, is constantly monitored. A malfunction of the sensor or of the evaluation circuit is immediately discovered by the diminution or attenuation of the electrical response signal to an infrared radiation pulse, and triggers a malfunction signal. Likewise, any attempt to sabotage the detector and to render the same insensitive to the detection of an intruder, e.g., by spraying the closure window or the entrance window of the housing with a spray which is impermeable or opaque to infrared radiation, is signalled in the same way as a malfunction.
In order to be able to distinguish a genuine alarm condition caused by an intruder from a malfunction in such previously known detectors, each sensor must be differently irradiated and the evaluation circuit must be able to evaluate and to display the two types of irradiation individually. For this purpose, either the radiation of the additional radiation source can be modulated in a specific manner and the evaluation circuit tuned to such modulation, which requires considerable expenditure in circuitry, or the optical arrangement is set to generate a number of restricted reception fields, as is known e.g. from the U.S. Pat. No. 3,703,718, the U.S. Pat. No. 4,058,726 or the European published Patent No. 25,188, published Jan. 26, 1983, and the evaluation circuit detects specifically and selectively a change in the irradiation of the sensor caused by movement of a burglar through such a reception field or zone and only gives an alarm signal in circumstances in which this change in irradiation has a specific predetermined form. This also requires considerable expenditure.
On the other hand, an infrared intrusion detector is known, from the U.S. Pat. No. 4,339,748 and other publications, in which the infrared sensor is designed as a dual sensor having two sensor elements connected in opposition to one another or antiparallel. On account of the small spatial displacement or separation of the two sensor elements in relation to one another, each optical element accordingly generates a pair of two closely adjacent reception zones, which are sequentially traversed by a burglar with a small temporal difference. As a result of the differential circuit connection of the two sensor elements, in the event of an alarm the evaluation circuit accordingly receives at least one each of a positive pulse and a negative pulse in rapid sequence. These pulses can be evaluated in a simple manner for generating an alarm signal, e.g., by means of a time gate or time window circuit, such evaluation in fact taking place independently of other signals.
In the case of such an infrared intrusion detector equipped with a dual sensor, the use of an additional radiation source directly irradiating the sensor for malfunction or sabotage supervision would however be ineffective, since the additional radiation source would uniformly irradiate the two sensor elements and the output signal of the differential circuit would accordingly be zero, and a malfunction or an attempt at sabotage could therefore not be detected.